Re: [PATCH v3 4/9] KVM: MMU: introduce SPTE_ALLOW_WRITE bit

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On Fri, Apr 20, 2012 at 04:18:47PM +0800, Xiao Guangrong wrote:
> This bit indicates whether the spte is allow to be writable that
> means the gpte of this spte is writable and the pfn pointed by
> this spte is writable on host
> 
> Signed-off-by: Xiao Guangrong <xiaoguangrong@xxxxxxxxxxxxxxxxxx>
> ---
>  arch/x86/kvm/mmu.c |   13 ++++++-------
>  1 files changed, 6 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> index e70ff38..dd984b6 100644
> --- a/arch/x86/kvm/mmu.c
> +++ b/arch/x86/kvm/mmu.c
> @@ -145,7 +145,8 @@ module_param(dbg, bool, 0644);
>  #define CREATE_TRACE_POINTS
>  #include "mmutrace.h"
> 
> -#define SPTE_HOST_WRITEABLE (1ULL << PT_FIRST_AVAIL_BITS_SHIFT)
> +#define SPTE_HOST_WRITEABLE	(1ULL << PT_FIRST_AVAIL_BITS_SHIFT)
> +#define SPTE_ALLOW_WRITE	(1ULL << (PT_FIRST_AVAIL_BITS_SHIFT + 1))
> 
>  #define SHADOW_PT_INDEX(addr, level) PT64_INDEX(addr, level)
> 
> @@ -1177,9 +1178,8 @@ static int kvm_set_pte_rmapp(struct kvm *kvm, unsigned long *rmapp,
>  			new_spte = *sptep & ~PT64_BASE_ADDR_MASK;
>  			new_spte |= (u64)new_pfn << PAGE_SHIFT;
> 
> -			new_spte &= ~PT_WRITABLE_MASK;
> -			new_spte &= ~SPTE_HOST_WRITEABLE;
> -			new_spte &= ~shadow_accessed_mask;
> +			new_spte &= ~(PT_WRITABLE_MASK | SPTE_HOST_WRITEABLE |
> +				      shadow_accessed_mask | SPTE_ALLOW_WRITE);

Each bit should have a distinct meaning. Here the host pte is being
write-protected, which means only the SPTE_HOST_WRITEABLE bit
should be cleared.
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